In a braking system, some elements are generally disposed in series between a brake operating member 900 and a vehicle wheel 902, for braking the vehicle upon operation of the brake operating member 900 by a vehicle operator, as schematically shown in FIG. 43. That is, there are disposed in series a brake operating mechanism 904, a booster 906, a master cylinder 908, a braking friction member 912 and a rotor 914.
The brake operating mechanism 904 is adapted to transmit to the booster 906 an operating force F which has been applied to the brake operating member 900. The booster 906 is adapted to boost the force received from the-brake operating mechanism 904, while utilizing a pressure, and transmit the boosted force to the master cylinder 908. As shown in FIG. 44, the booster 906 is able to boost the input force at a so-called "servo ratio" until a boosting limit has been reached, and is unable to boost the input force after the boosting limit has been reached. The master cylinder 908 has a pressurizing piston, which converts the output force of the booster 906 into a fluid pressure. The brake cylinder 910 has a brake piston, which converts the fluid pressure received from the master cylinder 908 into a force. The braking friction member 912 is forced by the output force of the brake cylinder 910, onto the rotor 914 (brake rotor, brake drum, etc.) rotating with the vehicle wheel 902 to be braked, and cooperates with the rotor 914 to brake the wheel 902, for thereby providing deceleration G of the vehicle body.
The braking system is required to cause the fluid pressure generated in the brake cylinder, to be as high as possible with a given brake operating force. This requirement is derived from an arrangement to reduce brake squeal and vibration. For instance, the arrangement employs a braking friction member made of a material which has a low friction coefficient or a large amount of compressive strain. This arrangement results in reducing a braking effect as represented by a ratio of the vehicle body deceleration G to the brake operating force F, as indicated in FIG. 45. To prevent the reduction in the braking effect due to the above arrangement, the brake cylinder is required to generate a fluid pressure as high as possible with a given brake operating force.
An example of an arrangement to increase the fluid pressure in the brake cylinder is to reduce the diameter of the pressurizing piston of the master cylinder. However, this arrangement results in reduction in the volume for pressing the pressurizing piston, which increases the required operating stroke of the pressurizing piston, causing another problem that the longitudinal dimension of the master cylinder is increased. Another example of the arrangement to increase the fluid pressure in the brake cylinder is to increase the servo ratio of the booster. This arrangement results in lowering the boosting point of the booster, as indicated in FIG. 46, so that the braking effect varies to a great extent while the operating force F is relatively small, causing another problem that the brake operating feel is deteriorated.
In summary, there is a limitation in an attempt to increase the fluid pressure in the brake cylinder with a given brake operating force, as long as the attempt relies on the master cylinder or booster. Thus, there has been a problem of difficulty to control as desired the relationship between the brake operating force and the fluid pressure in the brake cylinder.